1. Field of the Invention
This invention relates to fiber optic cable constructions, particularly a fiber optic cable suitable for use indoors at customer premises.
2. Discussion of the Known Art
Fiber-to-the-home (FTTH) services are usually supplied by so-called “drop” cables that route an optical fiber between a service provider's cable, and an optical network terminal (ONT) associated with a customer's residence. For typical North American installations, the enclosure of the ONT is mounted outside the residence and it protects the ONT from the outdoor environment. There is, however, a desire to deploy FTTH services in the following two new ways:
1. Service inside multi-dwelling units (MDUs), including high-rise buildings, low-rise buildings, and clustered developments of garden apartments. Here, the drop cables must run inside the building to an ONT mounted indoors at each residence.
2. Deployment of the ONT inside a home in single family dwellings. The current outdoor ONTs are large, bulky and aesthetically displeasing. Redesigning the ONT physically for placement inside the home would also allow it to be powered directly from the home's AC wiring, and a battery backup would be easier to deploy.
Both of the above scenarios require the fiber optic drop cable to be flame-retardant in order to comply with the applicable safety codes, and to limit the spread of fire in emergency situations. The drop cable should also be able to be installed inconspicuously inside the residence, and have robust optical and mechanical performance to allow for installation by workers having relatively low skill. Ideally, the cable should be capable of (a) being routed and installed in a manner similar to traditional copper wire cable used for decades to deliver telephony service, (b) wrapping tightly around sharp corners on walls, doors and wooden framing, (c) routing through basements and behind walls, and (d) being secured in place by way of conventional metal cable staples.
Optical drop cables under development for indoor use employ so-called bend insensitive singlemode fibers contained in thick, flame retardant jackets. For example, “ClearCurve™” optical fibers and drop cables are currently offered by Corning Inc., and have a 900 micron O.D. tight buffer surrounding the optical fiber for dimensional compatibility with existing factory mounted or splice-on optical connectors. The buffer material is believed to be a low cost flexible or semi-rigid poly(vinyl chloride) resin, thus providing an economical solution since the resin has a relatively low cost. The bend insensitive fiber allows the cable to be installed over a path having sharp bends with minimal signal attenuation, while a thick jacket prevents the fiber itself from being bent too sharply so as to preserve the fiber's specified optical performance. It has been discovered, however, that the known PVC buffers do not allow optimal protection against signal attenuation to be achieved in all FTTH applications.
Co-pending U.S. patent application Ser. No. 12/072,869 filed Feb. 28, 2008, and assigned to the assignee of the present application and invention, discloses a bend insensitive singlemode fiber having relatively low bend loss when bent to a radius of about 4 to 15 mm, and an effective aperture (Aeff) matched to that of standard singlemode fiber so as to enable the bend insensitive fiber to couple efficiently with a standard fiber when the two fibers are spliced to one another. The '869 application was published as US 2009/0060437 on Mar. 5, 2009.
Specifically, the fiber of the mentioned '869 application has a core and a cladding region for propagating light in a fundamental transverse mode. The cladding region includes (i) an outer cladding having a refractive index less than that of the core region, (ii) an annular pedestal region having a refractive index higher than that of the outer cladding and comparable to that of the core, (iii) an annular inner trench region disposed between the core and the pedestal region, the inner trench region having a refractive index less than that of the outer cladding, and (iv) an annular outer trench region disposed between the pedestal region and the outer cladding, the outer trench region having a refractive index less than that of the outer cladding. In addition, to suppress higher order transverse modes (HOMs), the pedestal region may be configured to couple at least one other transverse mode of the core resonantly to at least one transverse mode of the pedestal region.
For bend loss to be less than that of standard singlemode fiber at important operating wavelengths (e.g., 1300 nm, 1550 nm, and 1650 nm) for any bend radius in the range of about 4 to 15 mm, at least one of the inner and the outer trench regions (and preferably both) of the fiber of the '869 application provides a total contrast much higher than that of a standard singlemode fiber. In one embodiment disclosed in the '869 application, the contrast is given by:|ninner trench−ncore|>0.007, and/or  (Eq. 1)|nouter trench−ncore|>0.007.  (Eq. 2)For example, at least the inner trench-to-core contrast of Eq. 1, above, is approximately 0.008 to 0.020.
In addition, the interface between the outer cladding and the outer trench region should be located at a radial distance of approximately 17 to 23 μm from the center of the core, to enable the fiber to assume a bend radius of about 4 to 15 mm without significant loss. The refractive indicies of the core and the pedestal region should also be comparable; that is,|ncore−nped|<0.003
Example III of the '869 application defines an embodiment of a fiber having a dual trench, ring design as illustrated in FIG. 1B of the application, wherein the inner and the outer trench regions are both relatively deep and have essentially the same index depth. Specifically, the fiber has the following parameters and optical properties as taken from Tables 4 and 5 of the '869 application. The Δn values are relative to nouter cladding.
rcoreΔncoretshelfΔnshlfttriΔntritpedΔnpedttroΔntrorped4.40.00392.2−0.00044.7−0.00572.60.00397.2−0.005712.6
Bend LossStandardJumperat 1550 nmCable CutoffCable CutoffMFD(4.8 mm cable(22 m fiber (2 m fiberat 1310 nmbend radius)length)length)8.9 μm0.025 dB/turn1222 nm1253 nm
All relevant portions of the mentioned US 2009/0060437 patent application publication are incorporated by reference.
U.S. Pat. No. 5,684,910 (Nov. 4, 1997) discloses a buffered optical fiber having a strippable buffer layer. At least one layer of a protective coating is provided on the cladding of the fiber, and a plastics buffer layer of, e.g., nylon or poly(vinyl-chloride) (PVC) with a thickness in the range of 0.008 to 0.014 inch jackets the coated fiber. A boundary layer comprised of, e.g., an acrylate copolymer material is disposed between the fiber coating and the buffer layer, and has a thickness greater than about one-twenty-fifth the thickness of the buffer layer. If stiffer type materials such as low-smoke poly(vinyl-chloride)(LSPVC) or fire-retardant polypropylene (FRPP) are used for the buffer layer, then the boundary layer may be as thin as about 0.0005 inch, according to the patent.
Sufficient adhesion exists between the plastics buffer layer and the coated optical fiber to maintain the buffer layer in place during normal use of the fiber. The adhesion is low enough, however, so that the buffer layer can be removed without inflicting damage to the fiber when a reasonable mechanical stripping force is applied. All relevant portions of the mentioned '910 US patent are incorporated by reference.